Thick walled one-piece alloy structures are disclosed in U.S. Pat. Nos. 3,806,276 and 3,192,578. Laminated structures having thin walls capable of withstanding of high temperature impinging gases have heretofore been known. By way of example, such structures are disclosed in U.S. Pat. Nos. 4,245,769; 4,042,162; 4,004,056; 3,963,368; 3,950,114; 3,933,442; 3,810,711; 3,732,031; 3,726,604; 3,698,834; 3,700,418; 3,644,059; 3,644,060; 3,619,082; 3,616,125; 3,610,769; 3,606,572; 3,606,572; 3,606,573; 3,584,972; 3,527,544; 3,529,905 and 3,497,277. The thin walls of such structures are laminated to another thin wall or to a substantially thicker structure by brazing, welding or otherwise bonding. The laminating process involves high temperature brazing, Welding or bonding materials that directly affect the alloy or otherwise limit the overall high temperature performance of the structure. Further, these thin wall layers often have holes formed therein by mechanical means or etching which is time consuming, labor intensive and expensive. Although these laminated thin wall structures are capable of withstanding impinging gases at temperatures higher than the melting point of the alloys which the structures are made from, the process of making the structures is time consuming, labor intensive and extremely expensive.
Many prior art methods of casting hollow structures utilize ceramic cores. It has been generally accepted that these core must have a density sufficiently low enough such that the core is compressive so that it gives as molten alloy solidifies around the core. It has generally been accepted that if the core has a density above 60 to 70 percent, the core will be crushed and broken by molten alloy which solidifies around it. It has also generally been accepted that cores having a thickness less than 0.03 inches with such low density less would be crushed and broken during casting. The density of prior art ceramic cores ranges from about 50 to about 60 percent.
Although 100 percent quartz rods having 100 percent density have been used, such use has been limited to making bent and straight holes or central passageways. Heretofore, a high density ceramic core (above 70 to 99 percent plus density) has not been used to make a radial passageway. Generally, in turbine engine components such as turbine blades, such radial passageways parallel the outer thin wall of the component or turbine blade.
It is generally accepted that the use of a high density material for a large core will cause cracks to form in the metal. As molten alloy solidifies around a large high density core, the metal shrinks faster than the core and will break due to the high density core. Thus, those skilled in the art use low density cores to compensate for the fast rate of shrink of the molten metal and to prevent the metal from breaking.
Another problem recognized by those skilled in the art is the problem of shape distortion during casting. Heretofore, it has been generally accepted that this shape distortion of the casted part is caused by what is known as "mold buckle". This "mold buckle" occurs in the process of building up the shell around the core and pattern. If one of the successive shell layers does not sufficiently dry, the layer moves away from the pattern causing the mold to "buckle" and causes a distorted casting shape. Heretofore, it was not recognized that casting shape distortion could be caused by shell creep.
In prior art methods of making laminated thin wall structures such as gas turbine blades, the thin walls are provided by metal which has been cold rolled to a very thin thickness. The cold-rolled metal is then etched or machined to provide small holes in the surface thereof. The small holes provide a cooling air film over the thin wall as the gas turbine blade is impinged with hot gases. This cold-rolled metal must be formed and bonded (or welded sufficiently to provide heat transfer from the thin wall to the main body of the blade) in a curved shape to produce the outer wall of a turbine blade. The forming process may result in the distortion of the holes in the wall. If the holes are not properly positioned, or the metal not sufficiently bonded, it is possible to develop hot spots at certain sections on the blade which would be undesirable and would limit production yields. Further, the cold-rolled material must be later heat treated which also could possibly result in varying heat transfer properties across the surface of the blade which also would be undesirable.
Other casting problem are caused by ceramic cores which are extremely brittle and fragile. These problems increase with decreasing thickness and density.
Heretofore, there has been a need for single-cast, high-temperature, thin wall hollow structures and means for making the same which is quick, relatively inexpensive and not labor intensive. A means for satisfying this need has heretofore escaped those skilled in the art.